The present invention relates to ellipsometry. More particularly, the present invention pertains to imaging ellipsometry.
Ellipsometry is an optical technique that uses polarized light to probe the dielectric properties of a sample. The most common application of ellipsometry is the analysis of very thin films. Through the analysis of the state of polarization of the light that interacts with the sample, ellipsometry can yield information about such films. For example, depending on what is already known about the sample, the technique can probe a range of properties including the layer thickness, morphology, or chemical composition.
Generally, optical ellipsometry can be defined as the measurement of the state of polarized light waves. An ellipsometer measures the changes in the polarization state of light when it interacts with a sample. The most common ellipsometer configuration is a reflection ellipsometer, although transmission ellipsometers are sometime used. If linearly polarized light of a known orientation is reflected or transmitted at oblique incidence from a sample surface, then the resultant light becomes elliptically polarized. The shape and orientation of the ellipse depend on the angle of incidence, the direction of the polarization of the incident light, the wavelength of the incident light, and the Fresnel properties of the surface. The polarization of the light is measured for use in determining characteristics of the sample. For example, in one conventional null ellipsometer, the polarization of the reflected light can be measured with a quarter-wave plate followed by an analyzer. The orientation of the quarter-wave plate and the analyzer are varied until no light passes though the analyzer, i.e., a null is attained. From these orientations and the direction of polarization of the incident light, a description of the state of polarization of the light reflected from the surface can be calculated and sample properties deduced.
Two characteristics of ellipsometry make its use particularly attractive. First, it is a nondestructive technique, such that it is suitable for in situ observation. Second, the technique is extremely sensitive. For example, it can measure small changes of a film down to sub-monolayer of atoms or molecules. For these reasons, ellipsometry has been used in physics, chemistry, materials science, biology, metallurgical engineering, biomedical engineering, etc.
As mentioned above, one important application of ellipsometry is to study thin films, e.g., in the fabrication of integrated circuits. In the context of ellipsometry, a thin film is one that ranges from essentially zero thickness to several thousand Angstroms, although this range can be extended in many cases. The sensitivity of an ellipsometer is such that a change in film thickness of a few Angstroms can usually be detected. From the measurement of changes in the polarization state of light when it is reflected from a sample, an ellipsometer can measure the refractive index and the thickness of thin films, e.g., semi-transparent thin films. The ellipsometer relies on the fact that the reflection at a material interface changes the polarization of the incident light according to the index of refraction of the interface materials. In addition, the polarization and overall phase of the incident light is changed depending on the refractive index of the film material as well as its thickness.
Generally, for example, a conventional reflection ellipsometer apparatus, such as shown in FIG. 1, includes a polarizer arm 12 and an analyzer arm 14. The polarizer arm 12 includes a light source 14 such as a laser (commonly a 632.8 nm helium/neon laser or a 650-850 nm semiconductor diode laser) and a polarizer 16 which provides a state of polarization for the incident light 18. The polarization of the incident light may vary from linearly polarized light to elliptically polarized light to circularly polarized light. The incident light 18 is reflected off the sample 10 or layer of interest and then analyzed with the analyzer arm 14 of the ellipsometer apparatus. The polarizer arm 12 of the ellipsometer apparatus produces the polarized light 18 and orients the incident light 18 at an angle with respect to a sample plane 11 of the sample 10 to be analyzed, e.g., at some angle such as 20 degrees with respect to the sample plane 11 or 70 degrees with respect to the sample normal.
The reflected light 20 is examined by components of the analyzer arm 14, e.g., components that are also oriented at the same fixed angle with respect to the sample plane 11 of the sample 10. For example, the analyzer arm 14 may include a quarter wave plate 22, an analyzer 24 (e.g., a polarizer generally crossed with the polarizer 16 of the polarizer arm 12), and a detector 26. To measure the polarization of the reflected light 20, the operator may change the angle of one or more of the polarizer 16, analyzer 24, or quarter wave plate 22 until a minimal signal is detected. For example, the minimun signal is detected if the light 20 reflected by the sample 10 is linearly polarized, while the analyzer 24 is set so that only light with a polarization which is perpendicular to the incoming polarization is allowed to pass. The angle of the analyzer 24 is therefore related to the direction of polarization of the reflected light 20 if the minimum condition is satisfied. The instrument is xe2x80x9ctunedxe2x80x9d to this null (e.g., generally automatically under computer control), and the positions of the polarizer 16, the analyzer 24, and the incident angle 13 of the light relative to the sample plane 11 of the sample 10 are used to calculate the fundamental quantities of ellipsometry: the so called Psi, delta (xcexa8, xcex94) pair given by:             r      p              r      s        =      tan    ⁢          xe2x80x83        ⁢          Ψ      ⁡              (                  ⅇ          jΔ                )            
where rp and rs are the complex Fresnel reflection coefficients for the transverse magnetic and transverse electrical waves of the polarized light, respectively. From the ellipsometry pair (xcexa8, xcex94), the film thickness (t) and index of refraction (n) can be determined. It will be recognized that various ways of analyzing the reflected light may be possible. For example, one alternative is to vary the angle of the quarter wave plate and analyzer to collect polarization information.
Although many different types of ellipsometers exist, they have various shortcomings. For example, many are not suitable for characterizing samples that have very small transverse features. The smallest spot a conventional ellipsometer can measure is determined by the beam size, usually on the order of hundreds of microns. This essentially limits its application to samples with large and uniform interface characteristics. Resolution of an image produced by imaging ellipsometers is typically inadequate and improvement is necessary.
Advances in microelectronic fabrication are rapidly surpassing current capabilities and metrology. In order to enable future generations of microelectronics, some specific metrology capabilities must be developed. One of the key challenges is to measure the properties of complex layers of extremely thin films or submicron lateral dimensions.
Several systems have been developed to attack the above shortcomings. For example, to resolve the suitability of ellipsometers to characterize samples that have small transverse features, a microscope objective lens in a conventional ellipsometer has been used. For example, the microscope objective lens has been the basis for several ellipsometry methods including spatially resolved ellipsometry (SRE), image scanning ellipsometry (ISE), and dynamic imaging micro-ellipsometry (DIM). However, such methods and systems also have drawbacks.
With respect to spatially resolved ellipsometry, such techniques can measure small features, but they are typically too time consuming for many applications because the sample has to be measured point by point. Such a time consuming process makes this system highly undesirable for many applications.
With respect to ellipsometry systems that perform image scanning ellipsometry and dynamic imaging micro-ellipsometry, such systems usually use an imaging apparatus in an arm of a conventional ellipsometer to image the sample at a large incident angle. Such systems lead to different magnifications in two directions, which result in a distortion of an image being produced. A scanning mechanism or other complicated optical system is thus required to correct such distortion. Further, the slant or incident angle of the light relative to the sample plane also limits the use of the highest numerical aperture objective lenses, which, in turn, limits the achievable resolution of such systems.
Imaging ellipsometry according to the present invention is presented which characterize a sample with high resolution. The imaging ellipsometry described herein can perform accurate measurements with high speed and high resolution using a very simplified apparatus. Generally, to achieve high resolution and form an image, an objective lens (e.g., a high numerical aperture objective lens) is used. Polarization effects due to Fresnel reflection with a high numerical aperture objective lens are used as a measurement signal in the imaging ellipsometry according to the present invention.
An ellipsometry apparatus according to the present invention includes an objective lens having a focal plane at which a sample plane of a sample is positioned. An illumination source provides incident light normal to the sample plane. The incident light includes linearly polarized light incident on the objective lens. The objective lens focuses the incident light onto the sample. At least a portion of the focused incident polarized light is reflected by the sample resulting in reflected light. A spatial filter modifies at least a portion of the incident light and the reflected light. An analyzer portion is used to generate polarization information based on the reflected light.
In various embodiments of the apparatus, the illumination source may be a fiber illuminator, the objective lens may be a high numerical aperture objective lens having a numerical aperture in the range of 0.5 to less than 1, and the spatial filter may be positioned adjacent the objective lens in an actual plane of the exit pupil thereof or may be positioned in a conjugate plane of the exit pupil of the objective lens.
In another embodiment of the apparatus, the analyzer portion includes a rotatable quarter wave plate, an analyzer, a lens, and a detector, e.g., a charge coupled device array detector. The rotatable quarter wave plate, the analyzer, and the lens are positioned such that the reflected light passes through the rotatable quarter wave plate and the analyzer. Further, the reflected light is focused onto the detector by the lens.
In another embodiment of the invention, the apparatus further comprises a beam splitter that passes the linearly polarized light normal to the focal plane and incident on the objective lens. Further, the beam splitter diverts the reflected light to the analyzer portion.
An ellipsometry method according to the present invention for use in providing an image of at least a portion of a sample is also described. The method includes providing an objective lens having a focal plane at which a sample plane of the sample is positioned. A linearly polarized light normal to the sample plane and incident on the objective lens is further provided. The incident linearly polarized light is focused onto the sample and at least a portion of the focused incident polarized light is reflected by the sample, resulting in reflected light. At least a portion of the incident light or the reflected light is spatially filtered and polarization information is generated based on the reflected light.
In one embodiment of the method, the linearly polarized light normal to the sample plane incident on the objective lens is provided by providing light from an extended source, collimating the light, and linearly polarizing the collimated light. In other embodiments of the method, the high numerical aperture objective lens may have a numerical aperture in the range of 0.5 to less than 1 and spatial filtering may use a spatial filter at an actual plane of an exit pupil of the objective lens or a spatial filter at a conjugate plane of an exit pupil of the objective lens
In another embodiment of the method, the polarization information is generated by passing the reflected light through an analyzer portion comprising at least a rotatable quarter wave plate and an analyzer. At least the rotatable quarter wave plate is rotated to at least two angular positions. At least two polarization images corresponding to the at least two angular positions are detected.
In additional embodiments for the generation of polarization information, an image may be generated using a ratio or difference of the at least two polarization images. Further, the analyzer may also be rotated to one or more positions with corresponding additional polarization images being used for the generation of the polarization information.
In yet a further embodiment, the method may include providing the linearly polarized light normal to the sample plane incident on the objective lens with polarization states that are at xc2x145xc2x0 with respect to an incident plane of the linearly polarized light using a polarization converter. Further, generation of the polarization information based on the reflected light may be performed using a polarization device matched to the polarization converter.
In yet another embodiment of the method, the spatial filtering may be provided by using a spatial filter configured such that the polarization state of the light that is modified thereby is aligned at 45xc2x0 with respect to an incident plane of the linearly polarized light incident on the objective lens. In such an embodiment, the spatial filter may be synchronously rotated, with a rotatable quarter wave plate and an analyzer to generate a plurality of polarization images for use in generating polarization information.
Yet further, another embodiment of the method according to the present invention includes providing linearly polarized light by providing light such that an illumination line is focused on the sample. The illumination line is swept across the sample.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.